Overriding Methods & Class Hierarchies

Overriding Methods & Class Hierarchies
November 16, 2007 ComS 207: Programming I (in Java) Iowa State University, FALL 2007 Instructor: Alexander Stoytchev © 2004 Pearson Addison-Wesley. All rights reserved

Quick Review of Last Lecture
© 2004 Pearson Addison-Wesley. All rights reserved

The protected Modifier
Visibility modifiers affect the way that class members can be used in a child class Variables and methods declared with private visibility cannot be referenced by name in a child class They can be referenced in the child class if they are declared with public visibility -- but public variables violate the principle of encapsulation There is a third visibility modifier that helps in inheritance situations: protected © 2004 Pearson Addison-Wesley. All rights reserved

The protected Modifier
The protected modifier allows a child class to reference a variable or method directly in the child class It provides more encapsulation than public visibility, but is not as tightly encapsulated as private visibility A protected variable is visible to any class in the same package as the parent class The details of all Java modifiers are discussed in Appendix E Protected variables and methods can be shown with a # symbol preceding them in UML diagrams © 2004 Pearson Addison-Wesley. All rights reserved

The super Reference Constructors are not inherited, even though they have public visibility Yet we often want to use the parent's constructor to set up the "parent's part" of the object The super reference can be used to refer to the parent class, and often is used to invoke the parent's constructor © 2004 Pearson Addison-Wesley. All rights reserved

The super Reference A child’s constructor is responsible for calling the parent’s constructor The first line of a child’s constructor should use the super reference to call the parent’s constructor The super reference can also be used to reference other variables and methods defined in the parent’s class © 2004 Pearson Addison-Wesley. All rights reserved

super super super this this this

Modified Book Example See Words2.java (page 445)
See Book2.java (page 446) See Dictionary2.java (page 447) © 2004 Pearson Addison-Wesley. All rights reserved

Chapter 8 Sections 8.1 & 8.2

Overriding Methods A child class can override the definition of an inherited method in favor of its own The new method must have the same signature as the parent's method, but can have a different body The type of the object executing the method determines which version of the method is invoked © 2004 Pearson Addison-Wesley. All rights reserved

Overriding A method in the parent class can be invoked explicitly using the super reference If a method is declared with the final modifier, it cannot be overridden The concept of overriding can be applied to data and is called shadowing variables Shadowing variables should be avoided because it tends to cause unnecessarily confusing code © 2004 Pearson Addison-Wesley. All rights reserved

Overloading vs. Overriding?

Method Overloading The compiler determines which method is being invoked by analyzing the parameters [signature 1] tryMe: int float tryMe(int x) { return x ; } [signature 2] tryMe: int, float float tryMe(int x, float y) { return x*y; } © 2004 Pearson Addison-Wesley. All rights reserved

Method Overriding Same Signatures Different Method Bodies
public class Parent { public float tryMe(int x) return x ; } Same Signatures public class Child extends Parent { public float tryMe(int x) return x*x; } Different Method Bodies © 2004 Pearson Addison-Wesley. All rights reserved

Overloading vs. Overriding
Overloading deals with multiple methods with the same name in the same class, but with different signatures Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature © 2004 Pearson Addison-Wesley. All rights reserved

Overloading vs. Overriding
Overloading lets you define a similar operation in different ways for different parameters Overriding lets you define a similar operation in different ways for different object types © 2004 Pearson Addison-Wesley. All rights reserved

Overriding Example See Messages.java (page 450)
See Thought.java (page 451) See Advice.java (page 452) © 2004 Pearson Addison-Wesley. All rights reserved

Chapter 8 Section 8.3

Class Hierarchies A child class of one parent can be the parent of another child, forming a class hierarchy Business KMart Macys ServiceBusiness Kinkos RetailBusiness © 2004 Pearson Addison-Wesley. All rights reserved

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Animals Class Hierarchy

Class Hierarchies Two children of the same parent are called siblings
Common features should be put as high in the hierarchy as is reasonable An inherited member is passed continually down the line Therefore, a child class inherits from all its ancestor classes There is no single class hierarchy that is appropriate for all situations © 2004 Pearson Addison-Wesley. All rights reserved

Employee Class Hierarchy

The Object Class A class called Object is defined in the java.lang package of the Java standard class library All classes are derived from the Object class If a class is not explicitly defined to be the child of an existing class, it is assumed to be the child of the Object class Therefore, the Object class is the ultimate root of all class hierarchies © 2004 Pearson Addison-Wesley. All rights reserved

The Object Class The Object class contains a few useful methods, which are inherited by all classes For example, the toString method is defined in the Object class Every time we define the toString method, we are actually overriding an inherited definition The toString method in the Object class is defined to return a string that contains the name of the object’s class along with some other information © 2004 Pearson Addison-Wesley. All rights reserved

Object – the mother of all objects in Java

The Object Class The equals method of the Object class returns true if two references are aliases We can override equals in any class to define equality in some more appropriate way As we've seen, the String class defines the equals method to return true if two String objects contain the same characters The designers of the String class have overridden the equals method inherited from Object in favor of a more useful version © 2004 Pearson Addison-Wesley. All rights reserved

Exceptions Class Hierarchy

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Abstract Classes An abstract class is a placeholder in a class hierarchy that represents a generic concept An abstract class cannot be instantiated We use the modifier abstract on the class header to declare a class as abstract: public abstract class Product { // contents } © 2004 Pearson Addison-Wesley. All rights reserved

Abstract Classes An abstract class often contains abstract methods with no definitions (like an interface) Unlike an interface, the abstract modifier must be applied to each abstract method Also, an abstract class typically contains non- abstract methods with full definitions A class declared as abstract does not have to contain abstract methods -- simply declaring it as abstract makes it so © 2004 Pearson Addison-Wesley. All rights reserved

Abstract Classes The child of an abstract class must override the abstract methods of the parent, or it too will be considered abstract An abstract method cannot be defined as final or static The use of abstract classes is an important element of software design – it allows us to establish common elements in a hierarchy that are too generic to instantiate © 2004 Pearson Addison-Wesley. All rights reserved

Interface Hierarchies
Inheritance can be applied to interfaces as well as classes That is, one interface can be derived from another interface The child interface inherits all abstract methods of the parent A class implementing the child interface must define all methods from both the ancestor and child interfaces Note that class hierarchies and interface hierarchies are distinct (they do not overlap) © 2004 Pearson Addison-Wesley. All rights reserved

Visibility Revisited It's important to understand one subtle issue related to inheritance and visibility All variables and methods of a parent class, even private members, are inherited by its children As we've mentioned, private members cannot be referenced by name in the child class However, private members inherited by child classes exist and can be referenced indirectly © 2004 Pearson Addison-Wesley. All rights reserved

Visibility Revisited Because the parent can refer to the private member, the child can reference it indirectly using its parent's methods The super reference can be used to refer to the parent class, even if no object of the parent exists © 2004 Pearson Addison-Wesley. All rights reserved

You can use jGrasp to draw diagram like this one

Designing for Inheritance
As we've discussed, taking the time to create a good software design reaps long-term benefits Inheritance issues are an important part of an object-oriented design Properly designed inheritance relationships can contribute greatly to the elegance, maintainability, and reuse of the software Let's summarize some of the issues regarding inheritance that relate to a good software design © 2004 Pearson Addison-Wesley. All rights reserved

Inheritance Design Issues
Every derivation should be an is-a relationship Think about the potential future of a class hierarchy, and design classes to be reusable and flexible Find common characteristics of classes and push them as high in the class hierarchy as appropriate Override methods as appropriate to tailor or change the functionality of a child Add new variables to children, but don't redefine (shadow) inherited variables © 2004 Pearson Addison-Wesley. All rights reserved

Inheritance Design Issues
Allow each class to manage its own data; use the super reference to invoke the parent's constructor to set up its data Even if there are no current uses for them, override general methods such as toString and equals with appropriate definitions Use abstract classes to represent general concepts that lower classes have in common Use visibility modifiers carefully to provide needed access without violating encapsulation © 2004 Pearson Addison-Wesley. All rights reserved

Restricting Inheritance
The final modifier can be used to curtail inheritance If the final modifier is applied to a method, then that method cannot be overridden in any descendent classes If the final modifier is applied to an entire class, then that class cannot be used to derive any children at all Thus, an abstract class cannot be declared as final These are key design decisions, establishing that a method or class should be used as is © 2004 Pearson Addison-Wesley. All rights reserved

Overriding Methods & Class Hierarchies

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